1. Field of the Invention
This invention relates generally to aerodynamic wing study and specifically to a device for inducing flutter in an aircraft wing during structural dynamic testing thereof.
2. Description of the Related Art
Flight flutter and structural dynamic testing of aircraft generally requires a method of exciting the structure beyond the use of free stream turbulence or "stick raps" by the pilot. Aircraft control surfaces can provide such excitation if the flight control actuator has sufficient output over the frequency range of interest. Since flight control actuators are usually not designed for high frequency operation, some form of external exciter system is typically used. The two primary types of exciters are inertial and aerodynamic.
Inertial exciters are unbalanced masses driven by either an hydraulic or an electric motor. The primary disadvantages of inertial exciters are their large size and their heavy weight. Correspondingly, large motors are required to drive the large masses of such inertial exciters.
Aerodynamic exciters are external lifting surfaces which are pitch-oscillated to obtain the required excitation forces. Aerodynamic vanes on small surfaces of the wing are quite efficient in generating the dynamic forces required for flight flutter tests. Typically, they require hydraulic actuation because of the relatively large power required to overcome aerodynamic and inertial loads. This requirement for hydraulic power adds a considerable complication to the installation of such a system. Quite often, the cost becomes prohibitive.
U.S. Pat. No. 3,552,192 was issued to Grosser on Jan. 5, 1971, for a rotary excitation device. In the flutter exciter system of Grosser, a rotating aerodynamic vane is mounted adjacent to the surface of the outer tip of an aircraft wing or horizontal stabilizer at right angles to the direction of flight. Grosser's excitation device comprises a vane which rotates at a constant rate about its mid-chord axis. Because the chordwise center of pressure of the rotating vane changes with the angular position of the vane during each rotation cycle, the torque and consequently the power required to drive the system can become a major deterrent in implementing the concept of Grosser's flutter exciter system.
Therefore, it remains a problem in the prior art technology to provide a flutter exciter system which imposes minimal power demands on existing hydraulic or electrical systems of the aircraft. Also. it remains a problem to make a completely self-contained flutter exciter unit which may be simply mounted to any suitable hard point on the aircraft.